Distortion compensator



Aug. 9., .1927.

1,638,536 I K. KUPFMULLER V DISTORTION COMPENSATOR Filed Feb. 1. 1924 2 Sheets-Sheet 2 K41 W T Patented naa za,

KARL KuPENZtiLL R, F BERLIN-ERIEDENAU, GERMANY, AssIeNoR To srEMENs a HALSKE, AK'rIENGEsELLsoHArr, or SIEMENSSTADT, NEAR BERLIN, GERMANY, A

CORPORATION GERMANY.

nIs'ro 'rIoN coM-rENsA'roR.

Application filed February 1, 1924, Serial No. 689,972, and in Germany October 5, 1923; November 27, 1928.

j the lines, which de ends on the value of the so-called cut-o frequency (0 of the loaded line,-

wherein L and C are the inductance and the capacity of the coil sections. In order to counteract the detrimental transient effects which occur when handling telephone service over very long distances, it is necessary to keep the cut-off frequency higher than is otherwise used, for instance by reducing the .inductance of the coil, L This involves, however, an increase in attenuation and thus an uneconomical utilization of the circuits. v

The object of the present invention is a balancing method for correcting the". distortion of the speech'currents caused by transient 0 phenomena. The application of this method thus rendersit possible to' carry on a converation successfully over any distance by lines loaded in the known manner from .economical points of view.

Figs. 1 to 3, 6 and? are curves used in explaining the invention. Fig. 4 shows a speech transmission system designed in accordance with the invention. Fig. 5 represents a section of the network used in Fig. 4. In order to explain the improved process, it is necessary to consider first of all the cause of this distortion. It is known that alternating currents, along smooth lines, are transmitted undistorted particularly higher frequencies. The alternating current of any frequency 0) reaches the distant end of the line, having the len th Z, the inductance Land the capacity per unit length in accordance with the known propagation 'Long lines being assumed, the initial alternating voltage V sin wt produces at the whereinu indicates the angle, and B the atv tenuation; At higher frequencies we have ax om/m (4) with suflicient acc'ufacy, or thus being propor- I tional to frequency and ,8 constant,- particularly if the effect of the dielectric losses is not taken into account; Since according to Fouriers theorem all. transient phenomena frequency variation on the physical quantities voltage or current, in steady-state condition, itmay be concluded: if an alternat-'- ing voltage V is applied to a network and 1 i in the line are determined by'the effect of A the voltageV between any two points of I the networkinthe steady state is given by Vx=V e sin (wt a) *(5) in which a tlonal to the frequency, the alternating cur rents will be transmittedwith a certain lag' of time, but Without distortion. Since the lag is equal at allfrequencies, it would be without. influence on the conversation across such a network; it produces only a delay in the arrival of the words heard in the reis constant and" a proporceiver. Examining to what extent the relation (5) is fulfilled in a longloaded line, we find that the quantity a increases in a higher rate than frequency. According to the theory of wave filters which, as is well known, may be applied with great accuracy to loaded lines, this quantity is given for a line of the length Z by where s is the distance between two coils and Z the length of the line. Hence we obtain for low frequencies approximately the equation '(4) in the form' The eneral attenuation curve results from the fol owing. It is well known that repeaters are constructed so as to compensate the steady-state distortion of the alternating I currents caused by the electrical properties of the line, particularly within the range of the voice frequencies. In long loaded lines provided with repeaters, the quantity '6 varies with frequency approximately as In the range o to Q which includes the important VOlCefleQlleIlshown in -Fig. 2.

nice, 6 remains to a large extent constant. Comparing various lengths, it will-be seen that the curves of the remainingattenuation differ essentially onl in the slope above and below in, and (D ractically, oscillations which lie considerably beyond this range are, however, not transmitted eyen in the case of comparatively short line lengths. Thus the efiect of frequency variation on the attenuation I) proves to be no source of distortion for the-voice currents in long lines. This distortion therefore is due to the difference of the quantity a equation (6), from the yalues in the equations (4;) and (7). In

ipractice it has hitherto been attempted to avoid this distortion by increasing the value 'w considerably. Closer examination will 9 show that in such a way the differences between 'the quantities given by the equations (6% and (7 are actually diminished.

he present. invention relates to suitable means for m'akingflthe difference a 'a= A small within the important frequency range, that is, forsupplementinga in such a man-. .ner that the new phase angle increases in the ratio of frequency; In this way the condi tion attached ,to the equation is fulfilled,

no matter to'what extent the line is loaded, so that all oscillations of the voice currents arrive with the same lag. Said means may be inserted in the line at one or several points, "for instance at the repeater stations.

the equations .and. (7) follows for the difference within the important frequency ,range by: developing the terms into.

7 series a line having thev cut-off frequency w =180O0, thecoil distance s=2 km. and the length Z=150 km., this relation gives the angle differences-represented in Fig. 3. In

the'sa'me figure a broken curvee. g. indicates those values up to which the angle differences A are supplemented according to thislinvention so thatthe total phase angle increases in the ratio of frequency. 3

Fig. 4. shows a part of the speech transmission system across a long loaded line according to the system of the invention. K

and K represent two cable sections, each of which connects two repeater stations. The repeaters of a four-wire system for instance are indicated by V.- The device .13 represents thead'ditionakset to be connected with the line according to this invention. This device is to be considered as a recurrentnetthe property that it work ofiltwo sections, the single sections of which are'shown in Fig. 5.- The sections of said recurrent network are of the s'o-called lattice type. Two of the four bridge branches are provided with two equal choking coils L, while the other two contain two equal condensersC. Such a network has uniformly allows all the network being as frequencies to pam and that its charsumed nonsdissi pative,

acteristic impedance is equal to V indo pendent of. frequency. The efiect of frequency variation on the phase angle on between initial and end voltage may be represented by the function which can easily be ascertained by the well known formulae of current distribution and v Fig. 7 shows how the phase difference A of a loaded cable, 300 km. in length and of the cut-off frequency w -16500, is supplemented by inserting a 10-sections network of the above described type with'the constant The characteristic impedance of the sup- If plementary network being equal to V6. it may easily be adaptedto the impedance of the loaded line. This impedance too is approximately non-inductive, as is well known,

and varies with frequency according to the relation to is the cut-off frequency of the loaded line,

Z the characteristic impedance of the equivalent smooth line. The root difiering essentially from 1 only for comparatively large o,-the energy losses between supplementary network and hne are negllble.

The supplementary network B may be placed, either at the end of the whole transmission system or it may be distributed between the repeater stations. In general :it will be preferable to correct, in the mannermasses I described, each repeater section by-its elf, as

the quality of transmission is independent; of thevlength of the communication with-'- out special provisions at the end station.

- What I claim as my invention and desire" to secure by Letters Patent'is:

1. ,The combination in a wavqtransmis sion system of a loaded line for the trans 4 mission of signals over long distances, with a localized compensating network which produces a phase displacement supp ementary to that produced by the .loaded'line, so that the I Y phase angle'between the initial voltage and 'the terminal voltage increases substantially in proportion tothe fre quency of-the trans,-

mitted waves. I ,y

2.;In combination m a transmission system with loaded lines for communication over long distances, recurrent'networks of the so-called lattice type, the sections of which consist of four-bridge branches, two

. of saidv branches consisting of two equal h plement the phase angle between the initial and the end voltage in such a manner that I chokingcoils, and the other two oftwo equal condensers, said networks supplementingthe phasean'gle between the initial and the end voltage-in such a manner that said phase angle increases approximately in" the ratio of frequency,

3. In combination in a transmission system with loaded llnes for the, transmission of signals over long distances, means to supquency of the desired range, and a it increases approximately in the ratio of two of saidbranches consisting of two equal choking coils and the other two of-two equal condensers, said networks" supplementing the phase angle between the initial and the end voltage in such amanner that said phase angleincreases approximately in the ratio- ;of frequency, said supplementing of the '-phase angle. taking place at the repeaterstations.

5. In combination, a lo aded line whose plot of phase distortion in angnlar-measure agalnstfr'equency is principally concave upward from zero tothe upper limiting type network fwhose corresponding; plot is concave downwardly, whereby the combination gives a plot which is approximately a straightline throughthe origin.

he ttiee -v 6. In combination, a loaded line having repeaters at appropriate. intervals, and with each repeater a lattice type network to compensate phase distortion on the correspond ing section ofthe line. l i

In testimony whereofI aflix my signature.-

Kan]; KUPFMULLER. 

